1. Field of the Invention
The present invention relates to heatpipes for cooling integrated circuits. More specifically, the present invention relates to a heatpipe that utilizes a composite containing graphite fibers in a configuration which approximates a hemisphere.
2. Summary of the Prior Art
Referring to FIG. 1, a cross-sectional view of a heatpipe of the prior art is shown. Heatpipes 10 are well known for their use in cooling integrated circuits. The essence of a heatpipe 10 is to provide an evaporator in close proximity to a high power integrated circuit so that heat from the integrated circuit may be turned into vapor and rapidly carried away from the heat source.
A heatpipe 10 is basically comprised of two regions: the condenser 11 and the evaporator 12. The evaporator 12 is located either next to or on top of the integrated circuit 13 (hereinafter sometimes referred to as "die 13"). A thermal spreader 14 is sometimes placed between the die 13 and the evaporator 12. The thermal spreader 14 may be made of copper (Cu), molybdenum, ceramic or other material and serves to increase the surface area over which heat from the die 13 contacts the evaporator 12.
A fluid 16 is provided in the evaporator 12. Heat propagates from the die 13 to the evaporator 12 where it heats the fluid 16 bringing about boiling. The boiling fluid absorbs heat because a phase change from liquid to vapor requires heat energy. Fluid vapor is transported up the heatpipe 10 away from the heat source (the die 13). The vapor transfers heat to the condenser 11. At the condenser 11, the heat is transferred to the heatpipe fins 17 where it is removed to the environment. The vapor condenses in the condenser 11 and the condensing liquid flows back down to the evaporator where it may again be boiled, thereby repeating the process.
The configuration of the heatpipe 10, utilizing vapor to carry away heat, is used because heat transfer via mass transfer in a vapor can be more efficient than conduction in a solid. If, for example, instead of being hollow, the heatpipe 10 were made of solid Cu, then the heat would propagate a limited distance into the Cu and no farther. The result is that the heat source (13) would run hotter, which is undesirable.
A heatpipe 10 may also have a wick 18. A wick 18 is usually a woven mat, made of Cu or some other material, which moves condensing fluid from the condenser region 11 to the evaporator 12 by capillary action. The use of a wick 18 permits a heatpipe 10 to be positioned arbitrarily with respect to gravity. Surface tension carries the condensing fluid along the wick 18 towards where the wick 18 is dry, thereby returning fluid to the evaporator 12 where it may be boiled again.
There are shortcomings to this prior art arrangement. One shortcoming is the rate at which heat is transferred from the evaporator surface 12a to the fluid 16. This is a function of the surface area of the evaporator surface 12a . The greater the surface area, the greater the rate of heat exchange. Although thermal spreaders 14 and finned evaporators have been used to effectively increase the area of the evaporator surface 12a which receives die 13 heat, they are of limited significance in efficiently providing heat transfer.